1. Field of the Invention
The present invention relates to an image sensing apparatus and more particularly to an image sensing apparatus for sensing a moving image.
2. Related Background Art
FIG. 6 is a block diagram showing the structure of a conventional X-ray image sensing apparatus for sensing a moving image by utilizing X-rays. In FIG. 6, reference numeral 101 represents an X-ray source for radiating X-rays, and reference numeral 102 represents a X-ray image sensing panel. The X-ray image sensing panel 102 has a plurality of photoelectric conversion elements disposed two-dimensionally and a driver circuit for driving these elements. An X-ray radiated from the X-ray source 101 passes through a subject 103 and becomes incident upon the X-ray image sensing panel 102 which detects the image of the subject. The X-ray passed through the subject 103 is converted into visible light by an phosphor (not shown) and then becomes incident upon the X-ray image sensing panel 102.
Reference numeral 104 represents an A/D converter for A/D converting a signal supplied from the X-ray image sensing panel 102, reference numeral 105 represents an FPN (fixed pattern noises) memory for storing FPN corrected values, and reference numeral 106 represents an FPN obtaining timing generation circuit for generating a timing signal for obtaining an FPN corrected value. When the FPN obtaining timing generating circuit 106 generates a timing signal, a switch 107 is turned on so that FPN is read from the A/D converter 104 into the FPN memory 105. Reference numeral 108 represents a subtractor for subtracting a corrected value read from the FPN memory 106 from an output of the A/D converter 104, reference numeral 109 represents a monitor for displaying a sensed image, and reference numeral 110 represents a recording medium for recording image data.
FIGS. 7A to 7C are timing charts illustrating an operation of the X-ray image sensing apparatus shown in FIG. 6. FIG. 7A shows the FPN obtaining timing signal supplied from the FPN obtaining timing generating circuit 106, and FIG. 7B shows an output from the A/D converter 104. At the start of the image sensing, the FPN obtaining timing signal shown in FIG. 7A is supplied to the switch 107 to turn it on, so that the FPN corrected value is read from the A/D converter 104 into the FPN memory 105. Thereafter, during the image sensing, the subtractor 108 subtracts the corrected value stored in the FPN memory 106 from an output of the A/D converter 104 as shown in FIG. 7C, so that the corrected image data with FPN being removed is supplied to the monitor 109 and recording medium 110.
With the conventional moving image sensing apparatus, since the FPN corrected value is stored in the FPN memory at the start of the image sensing, the FPN corrected value is fixed. Namely, the FPN corrected data cannot be obtained always during the moving image sensing. For this reason, for example, if the offset of an output amplifier (an output stage amplifier in the X-ray image sensing panel) changes with a power supply fluctuation, a temperature change or the like, a change in the offset appears directly in an output as shown in FIG. 7B. Therefore, as shown in FIG. 7C, a change in the offset appears directly in the corrected output and the image quality is degraded.